One limiting factor for the maximum power of a semiconductor laser diode design is the catastrophic optical damage (COD) limit at the output front facet of the laser diode. Essentially the very high optical power density, current density and carrier density interact with defects, non-radiative recombination centers, optical absorption areas and the semiconductor/coating/air interface to cause excess heating and eventually destructive failure. Various methods have been employed to increase the COD limit which will be discussed along with the new inventions.
Note that the COD generally occurs at the output front facet due to the higher optical power density relative to the rear facet. This invention applies primarily to the front facet window, however the same considerations as disclosed in this invention can also be applied to the rear facet. The front facet typically has a low reflectivity coating or coating stack deposited after an optional surface passivation. The rear facet is typically coated with a high reflectivity coating or coating stack. Also note that this window design applies to any high power semiconductor laser at any lasing wavelength including, but not limited to, single-mode lasers, multi-mode lasers, fiber-coupled lasers, distributed Bragg reflector (DBR) lasers and distributed feedback (DFB) lasers.
One of the first structures to improve COD is the use of a transparent window area 3 as shown in FIG. 1, which shows an output end 10 of a semiconductor laser in longitudinal cross-section. Basically a quantum well active region 2 is isolated from semiconductor/facet coating 6/air interface at front facet 5. This reduces the local optical absorption and heating near the front facet 5. Also typically the transparent window area 3 blocks most of injected current 1, which is reduced to a leakage current 11 considerably smaller than the injected current 1. The transparent window area 3 can be formed using a variety of methods including, but not limited to, local etch and regrowth, ion implantation, or diffusion intermixing.
Welch et al. (U.S. Pat. No. 4,845,725) disclose a preferred structure, which employs impurity induced disordering to smear the interfaces between active region and cladding layers producing a waveguide layer with increased bandgap and thus a transparent window region at the laser facet and a graded transverse refractive index profile. Window regions having transparent waveguide layers can be produced by impurity induced disordering, i.e. the diffusion of silicon, zinc, tin or other impurity through the semiconductor layers to form the window region.
The fabrication of the structure by Welch et al. entails several diffusion or implantation steps as well as crystallographic disordering, all of which are notorious for introducing non-radiative recombination centers. Since the introduced non-radiative recombination centers extend into the active layer of the laser, which is electrically pumped, they represent a serious laser degradation and reliability risk.
There are two problems with this design. There can be leakage current through the transparent window area 3 or, even worse, along the facet 5, which has non-radiative recombination centers, even with facet coating passivation 6. Also the interface 16 between the quantum well and transparent window typically will have defects and current flowing near those defects will recombine non-radiatively.
Another structure for improving COD uses a current blocking area, or unpumped structure 4, as shown in FIG. 2, which is a longitudinal cross-sectional view of an output end 20 of a semiconductor laser. In this case one or more blocking materials or layers 4 prevent current flow near the front facet 5 and therefore reduce non-radiative recombination near the front facet 5. The current blocking area 4 can be formed using a variety of methods including, but not limited to, patterned contact metal, a blocking insulator (such as silicon nitride, silicon dioxide, aluminum oxide, titanium oxide, etc.), a current blocking implant or diffusion, or current blocking semiconductor layers (such as etching off the contact layer or doing an etch and blocking re-growth).
Yu et al. (U.S. Pat. No. 6,373,875) disclose such a semiconductor laser structure incorporating a current blocking area 4, specifically to prevent current leakage near the facet 5, as illustrated in FIG. 2.
There are two problems with this design. There will be optical absorption in the quantum well active region 2 along the section 25 where it is not pumped to transparency. Also the semiconductor/facet coating 6/air interface at the front facet 5 will typically be a location of defects causing further optical absorption.
Kamejima (U.S. Pat. No. 4,759,025) discloses a semiconductor laser structure which attempts to resolve COD problems by using an intermixing technique. The laser structure is grown with thin multiple layers located at the intended active layer. The area to be pumped electrically by an electric current is thermally interdiffused by laser irradiation to form a mixed crystal exciting region having a band gap narrower than that of the surrounding layer, which is thus transparent to emitted light. In particular the non-interdiffused area near the laser facets becomes transparent to the laser emission. Also, the pumping current flows preferentially through the lower bandgap interdiffused area, thereby reducing exposure of the laser facets to the pumping current.
Kamejima's structure is unsuitable for high power lasers for a fundamental reason, however. To achieve high output power levels and high efficiency, a single or multiple quantum well (MQW) active layer consisting of one or more very thin semiconductor layers is generally preferred. The intermixing process on which Kamejima's structure depends destroys the quantum well structure.
An object of the present invention is to improve the laser structure for use at high optical power output levels by mitigating the adverse thermal effects in the vicinity of the laser facets such as recombination diverting some of the pumping current to non-radiative recombination centers and optical absorption.